Conventionally, a so-called closed circuit pumpless brake system of an ABS control device is known in which, in place of using a brake fluid recirculation pump when reducing pressure of a wheel cylinder during ABS control, brake fluid is temporarily stored in a reservoir provided in a conduit at a time of reducing pressure, and when pressure of a master cylinder is reduced along with release of a brake pedal, the brake fluid is recirculated from the reservoir to the master cylinder (Japanese Patent Laid-Open Publication No. Hei. 11-20645). In addition, this publication also discloses a so-called open circuit pumpless brake system in which, brake fluid that is discharged at the time of pressure decrease of the wheel cylinder, is directly recirculated to a master reservoir.
In the above mentioned pumpless ABS control device, for example, when a coefficient of friction on a road is low (e.g., when a road surface is prone to cause slipping), braking continues for a long time period if the brake pedal is depressed for a long time by a driver when a vehicle is travelling at a relatively high speed, there is a possibility that execution of accurate ABS control will cease.
In other words, when the aforementioned closed circuit is adopted in the pumpless ABS control device and the ABS control is continued for a long time, the reservoir becomes full. Therefore, it is no longer possible to further reduce brake fluid pressure. In this case, a master cylinder pressure in accordance with a depression amount of the brake pedal is applied to the wheel cylinder directly. Accordingly, it is possible that wheels may be locked.
In the case that the aforementioned open circuit is adopted in the pumpless ABS control device, there is no limit to a recirculation of fluid at the time of pressure decrease, as is the case with the closed circuit. However, a limit of the depression of the brake pedal corresponds to a limit for pressure increase of the master cylinder pressure. Namely, a stroke of the master cylinder is insufficient, and thus there is a possibility that it will become impossible to execute the ABS control over a long period due to the occurrence of bottoming-out of the master cylinder.
On the other hand, in Japanese Patent Laid-Open Publication No. Hei. 06-183330, a motor-driven brake device is disclosed in which a master cylinder pressure is generated by a piston of a master cylinder being directly acted upon and moved in accordance with a depression force of a brake pedal. In the motor-driven brake device, when the depression of the brake pedal is released, the master cylinder pressure is also released accordingly. Accordingly, when the ABS control is executed using the motor-driven brake device, it is essential for a driver to release depression of the brake pedal when pressure decrease control of the ABS control is executed. As a result, the motor-driven brake device is impractical.
In view of the foregoing circumstances, it is an object of the present invention to provide a motor-driven brake device in which insufficiency of a stroke of a master cylinder does not cause bottoming-out when an ABS control continues for a long time period, even if when a brake pedal is being depressed.
In addition, a further object of the present invention is to provide a motor-driven brake device that can suitably reduce a pressure of a wheel cylinder when an ABS control continues for a long time period, even if a brake pedal is being depressed.